This invention relates to a motor vehicle hub for a vehicle having an anti-lock braking system and in particular to a cast iron hub with an integral exciter ring.
In recent years, anti-lock braking systems (ABS) are being found on an increasing number of passenger motor vehicles. The ABS detects wheel rotation and acts to relieve braking force applied to the vehicle's wheels to prevent a lock-up condition from occurring. Wheel brake locking is undesirable since it can result in longer stopping distances and directional instability of the vehicle. The most advanced ABS designs employ a sensor at each wheel subject to ABS control. The sensors detect wheel rotation using various technologies including variable reluctance sensors and inductive pickups. Signals from the wheel rotation sensors are fed into an ABS computer which modulates the braking force.
The variable reluctance sensors and inductive pickup sensors typically use some type of tone wheel or exciter ring that rotates relative to the sensor in response to wheel rotation. Hereinafter, the term "exciter ring" will be used to refer to any such member, the rotation of which is detected by the sensors. One common form of a exciter ring is a cylindrical ring formed by a strap of steel or other material that is perforated to form a plurality of axially aligned slots through the strap. The strap is bent to form a ring with the two ends butt welded together and is press fit over a cylindrical surface of the wheel hub to rotate therewith. The sensor is mounted to a non-rotating component of the vehicle's suspension and spaced from the exciter ring.
There are, however, several problems associated with such an exciter ring. It is necessary to maintain a precise clearance between the sensor and the exciter ring on the order of 0.030-0.050 inches. If this dimension is not precisely maintained, the sensor, and hence the ABS, will not function as intended. With a press fit exciter ring on the hub there is a dimensional stack-up resulting in a radial run out and variability in the clearance between the exciter ring and the sensor. Another problem is that the hoop stresses formed in the exciter ring as a result of being press fit onto the hub can cause a failure of the butt weld and subsequent loss of the exciter ring. A further problem is created by corrosion of the exciter ring and/or the hub that results in the exciter ring separating from the hub. The first problem can result in improper functioning of the ABS while the second and third problems result in a total failure of the ABS.
Accordingly, it is an objective of the present invention to provide a hub and exciter ring assembly that overcomes the above problems.
The present invention overcomes the above problems by integrally forming the exciter ring as a part of the hub itself, rather than forming the exciter ring of a separate component press fit to the hub. One way to integrally form the hub and exciter ring is to machine the exciter ring into the hub. However, the magnetic permeability of the cast iron may not be sufficient for proper functioning of the sensor. To provide a magnetic permeability greater than that possible with the cast iron hub, a steel ring is first bonded to the hub by insert casting the ring into the hub. After casting, the hub is machined to expose the ring at the surface of the hub and the ring itself is machined to provide a uniform surface at the propoet location for producing the necessary clearance between the hub surface and sensor. Grooves are subsequently machined into the ring to form spaced teeth therebetween.
By insert casting of the steel band into the hub, the steel band is molecularly bonded to the cast iron. As a result, there is no butt weld necessary to maintain the steel band on the hub, nor is there a possibility of corrosion between the two materials that would result in the loosening of the band. Furthermore, by machining a cylindrical surface after casting, close tolerance and the roundness of the exciter ring can be ensured, resulting in a more precise clearance between the sensor and the exciter ring.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.